Note that the 2N2222 / 2N2907 (also PN2222 / PN2907)
transistors
will safely handle 8 to 10 times as much current as the
2N3904 / 2N3906 transistors.
Meanwhile, they only cost a few cents more per device -- you
may just want to buy the higher-current ones and simplify
your work space.

Transistor packaging
T0-92 is the most-common transistor
package BEAMers
will see. Unfortunately, there are multiple pin-out
configurations (standards are wonderful, there are so many
to choose from...). For bipolar transistors,
2 are the most common, depending on which type of transistor
is available locally (BC* transistors
are generally used in Europe and Australia, 2N* transistors
everywhere else):